Method and apparatus for producing a spectroscopic emission spectrum of a material



Nov. 9, 1965 a. K. WEHNER 3,217,162

METHOD AND APPARATUS FOR PRODUCING A SPECTROSCOPIG EMISSION SPECTRUM OF A MATERIAL Filed April 14, 1961 2 Sheets-Sheet l WATER INLET so 84 30v. D.C.

FIG. 3

IN VEN TOR. GOTTFRIED K. WEHNER TO MERCURY DIFFUSION PUMP I ATTORNEY Nov. 9, 1965 s. K. WEHNER 3,

METHOD AND APPARATUS FOR PRODUCING A SPECTROSCOPIC EMISSION SPECTRUM OF A MATERIAL Filed April 14, 1961 2 Sheets-Sheet 2 INVEN TOR.

GOTTFRIED K. WEHNER ATTORNEY United States Patent 3,217,162 METHOD AND APPARATUS FOR PRODUCING A SPECTROSCOPIC EMISSION SPECTRUM 65 A MATERIAL Gottfried K. Wehner, Minneapolis, Minn, assignor, by mesne assignments, to Litton Systems lino, Beverly Hills, Calif., a corporation of Maryiand Filed Apr. 14, 1961, Ser. No. 103,957 12 (Iiairns. ((1250- 195) The present invention relates to improvements in spectroscopic sources. More particularly, it pertains to apparatus and methods for bringing a material to be spectroscopically studied into a gaseous state by sputtering.

Spectroscopic source units for providing an emission spectrum of a material to be spectroscopically studied by exciting atoms of said material to high energy levels by heating said materials so as to convert them to a vapor state and then exciting the vaporized atoms in flame, arc, or spark sources are known. Also known the spectroscopic source units in which the material to be spectroscopically studied is evaporated in a vacuum from a little oven and then the atoms are excited in a high frequency discharge. To the best of my knowledge in all these known spectroscopic source units heat is used to convert the material to be analyzed into a gaseous form. When working with these known spectroscopic source units I have found it difficult to eliminate continuum background in the spectrum and to prevent diffusion in the bulk of the material to be analyzed.

Accordingly, one object of the present invention is to provide a spectroscopic source which eliminates continuum background in the spectrum.

A further object of this invention is to provide a spectroscopic source which eliminates diffusion in the bulk of the material to be analyzed.

Another object of this invention is to provide an emission spectrum in which the spectrum has the same composition as the material being spectroscopically analyzed.

Another object of the present invention is to provide a spectroscopic source whereby specific areas of a material can be analyzed.

Other objects and advantages in the invention will be apparent from the following description in which certain preferred embodiments of the invention are disclosed.

In the drawings which form a part of this application:

FIGURE 1 is a cross section view of a spectroscopic source unit according to the present invention; and

FIG. 2 is a sectional view of the spectroscopic source unit shown in FIG. 1; and

FIG. 3 is a schematic diagram of a circuit that may be used to operate the unit of FIGURE 1; and

FIG. 4 is a cross sectional view of the upper portion of the unit shown in FIGURE 1 showing a modification of said section; and

FIG. 5 is a cross sectional view of the modified upper portion of the unit shown in FIGURE 4.

In its broad aspects the invention herein disclosed includes the steps of subjecting a material which it is desired to spectroscopically study to controlled ionic bombardment and thereby sputter oil atoms of said material and then exciting said sputtered atoms in a low pressure high density plasma to higher energy levels. As is well known when atoms raised to energy levels higher than at which they are normally stable return to lower energy levels they give 011 energy which provides an emission spectrum of said material.

Referring now to the figures, FIGURE 1 depicts a spectroscopic source or tube having an upper chamber or envelope section 12 and a lower section or tank assembly 14. The tank assembly 14 has a conduit 15 connected thereto. The conduit 15 connects the interior of 3,217,162 Patented Nov. 9, 1965 the tank assembly 14 to a mercury diffusion pump (not shown).

The tank assembly 14 includes a first cylindrical wall 16, a second cylindrical wall 18, and a third cylindrical wall 20 located between and spaced from said first and second walls to form reservoirs 22 and 24 between said walls. The wall 18 forms a short length, large diameter passage 19 which connects with one end of the conduit 15. The reservoir 22 formed between the walls 16 and 20 is adapted to contain water while the reservoir 24 formed between the walls 20 and 18 is adapted to contain a mercury pool 26. A cathode spot anchor 27 is positioned in the mercury reservoir. The anchor 27 is a strip of molybdenum formed in a circle which has a diameter equal to the distance from the outside surface of the wall 16 to the inside surface of the wall 20. The strip of molybdenum can be in any form as long as it surrounds the point at which an igniter 52 comes in contact with the surface of the mercury pool 26.

The wall 20 has on its upper end a shoulder member comprising a shoulder portion 28, a radially extending flange portion 30, an upstanding wall portion 32, and an upstanding collar portion 34. The shoulder portion 28 is provided with a passage 35. The passage 35 provides means whereby gas can be admitted into the interior of the envelope 12. The upstanding wall portion 32 has an annular recess 36 therein. The recess 36 contains rubber O-ring seal 38. The envelope 12 is firmly and releasably positioned in the recess 36 on the O-ring seal 38. This connection between the tank assembly 14 and the envelope 12 provides a gas tight seal therebetween.

The envelope 12 can be a Pyrex glass bell jar. It is also possible to make the envelope 12 of other materials, such as quartz or metal. The envelope 12 has a reentrant cold trap cavity 39 formed therein which extends into the interior of said envelope 12. This cavity 39 is constructed so that it can receive cool materials such as liquid nitrogen. When filled with such cool materials, mercury ions in the envelope 12 stick to the surface of the cavity 39 when they come in contact therewith. Said surface is disposed in the interior of the envelope 12.

The interior of the envelope 12 is separated from the interior of the tank assembly 14 by a circular top. plate 40 having a centrally located hole 42. The plate 40 is disposed in a seat 44 formed in the shoulder portion 28; As shown the plate 40 is insulated from the shoulder portion 28.

The wall 18 extends upwardly toward said plate 40 so as to provide a passageway 46 between the mercury pool reservoir 24 and the passage 19 formed by said wall 18. The passage 19 has a large opening of short length which is adapted to connect the reservoir 24 to a mercury diffusion pump (not shown). The passage 19 is concentrically surrounded by the mercury pool reservoir 24.

A plurality of electrodes are suppo'rtedly connected to the tank assembly 14. The plurality of electrodes include a fine mesh graphite grid 50 positioned in the top plate hole 42, an igniter 52 adapted to be in contact with the mercury pool 26, an auxiliary anode 54 concentrically located with regard to the passage 19 between the mercury pool 26 and the grid 50, an anode 56 disposed in the envelope 12, and an electrode 58 adapted to hold a target in the envelope 12.

The grid 50 separates the tube 10 anode'space from the cathode space thereof. The grid 50 may be about 0.4 mm. thick having six holes per cm. of length, said holes being 1.3 mm. diameter as explained in pages 690-704 of vol. 102, No. 3 of The Physical Review. The grid 50 helps to stabilize the main discharge in the tube 10 and permits a considerable increase in plasma density within the anode space in the upper envelope 12 without utilization of undesirably high discharge currents and also permits an appreciable, yet simple, control of the velocity of accelerated beam electrons by variations on the grid potential. Located above the grid 50 is an Alnico magnet 59 which is held in place by knownmeans. The magnet 59 is provided with an annular passage 61.

Individual insulated covered electrical leads 60, 62, 64, 66, and 68 extend from the grid 50, the igniter 52, the auxiliary anode 54, the anode 56, and the target electrode 58, respectively, to the outside of the tube 10 through the tank assembly 14. Electric lead 69 is connected to the tank assembly 14. As indicated the electrical leads 60, 62, 64, 66, and 68 are insulated from the tank assembly by appropriate means such as glass insulators 70, 72, 74, 76, and 78, respectively. These insulators are held in place by appropriate means such as screws and O-rin-gs as is well known in the art in such a way that they are gas tight.

The electric leads 60, 62, 64, 66, 68, and 69 are connected to a ground 80 as shown in FIGURE 3. The electric lead 60 is connected to the ground 80 through DC. power supply 82 and a resistor 84 connected in series. The electric lead 69 which is electrically linked through the stainless steel tank 14 to the molybdenum strip 27 is connected directly to the ground 80. The electric lead 62 is connected to one terminal of a two way switch 86. A second terminal of the two way switch 86 is connected to ground through a DC. power supply 88 while a third terminal of the switch 86 is also connected to ground through a condenser 90. The switch 86 is normally in the position indicated by the solid line. When it is desired to activate the igniter 52 the switch 86 is momentarily moved to its dotted line position. The lead 64 is connected through a resistor 92 to the power supply 88. The common junction 94 between the resistor 92 and the power supply 88 is connected to one end of a variable resistor 96. The other end of the variable resistor 96 is connected to the lead 66. The lead 68 is linked through a DC. power supply 98 and a resistor 100 connected in series to a junction 102 between resistor 96 and lead 66.

Located inside of the envelope section 12 are channel members 104 and 106 (see FIGURE 2). The channel members 104 and 106 are provided with recesses 108 and 110 respectively. The recesses 108 and 110 are adapted to receive the edges of a curved metallic shutter member 112, and to permit said shutter to move up and down along one portion of the inner surface of the envelope 12. The shutter member 112 is made of magnetically attractable material so that when a magnet 114 is placed in close proximity to the outside surface of the envelope 12 as shown in solid lines on FIG. 1 and is then moved upwardly along the surface of the envelope 12 to the dotted line position 114a shown on FIG. 2 it will cause the shutter 112 to move upward to its dotted line position 112a. When the shutter 112 is in its dotted line position the portion of the envelope 12 which was formerly behind the shutter 112 provides an observation window 113 therein for viewing the inside of the envelope between the target material 58 and the magnet 59.

FIGURES 4 and illustrate another arrangement of a shutter and observation window whereby the space be tween the target material 58 and the magnet 59 can be observed. In this embodiment a shutter 116 is fixedly positioned between the observation window 117 and the space between the target material 58 and the magnet 59. Also fixedly positioned within the envelope 12 is a mirror 118. The mirror 118 is positioned so that one looking at said mirror through the observation window 117 located behind the shutter 116 can view through said mirror the space located between the target electrode 58 and the magnet 59. The line of sight through said window to said space is depicted by numeral 120.

p In atypical operation of the herein described spectroscopic source 14 a material 122 which one desires to provide an emission spectrum thereof is electrically connected to the electrode 58. The envelope 12 is then placed in position as shown in FIGURE 1. The mercury diffusion pump then lowers the pressure within the tank assembly 14 and the envelope 12 to about 1 micron. By having the passage 19 between the tank assembly 14 and the pump constructed to provide a short length large opening therebetween as shown herein 'one can achieve higher pumping speeds and cleaner discharge conditions within the tube 10.

After the pressure within the tank assembly 14 and the envelope 12 reaches the desired level a DC. discharge of approximately 3 amps at 15 volts voltage drop is ignited between the mercury pool 26 and the auxiliary anode 54 by means of a current pulse to the igniter 52. This current pulse is obtained by momentarily moving the switch 86 from its solid line position to its dotted line position. Since the main anode 56 is connected through resistor 96 to the DC. power supply 88, the main discharge of approximately 5 amps at 30 volts voltage drop between the pool 26 and the anode 56 can then be established. This causes the anode section of the tube, i.e., the space inside the envelope 12 above the grid 50, to become filled with a low pressure high density mercury plasma with a density of the order of 10 to 10 ions/ cm. The material 122 attached to the electrode 58 becomes surrounded by and/or immersed in this plasma.

The pressure of this gas discharge plasma around the material to be studied facilitates the removal of contaminants therefrom. Said contaminants are removed by subjecting the immersed material to preliminary ionic bombardment for a predetermined cleaning period. This is accomplished by making said material negative with respect to the anode 56 or the plasma.

After establishing sufiiciently clean condition for the material 122, said material is kept negative with respect to the anode or the plasma. This permits said material to be subjected to further ionic bombardment. This further ionic bombardment which can be controlled by proper application of potentials to the various elements of the unit shown in FIGURE 1 by a circuit such as show n FIG. 3 causes atoms of said material to be sputtered away from said material. This sputtering brings a portion of the material to be studied into a gaseous state which is one step in providing an emission spectrum of said material in accordance with the present invention.

I have found that in general sputtering rates can be more closely controlled than evaporation rates. In addition, I have found sputtering rates of a material to be more or less independent of the temperature of said material. One can therefore keep the material to be studied well below its melting temperature during sputtering and thus eliminate the need for containers to hold said materials When they melt.

The gas pressure in which the material 122 is immersed is kept low enough to keep the mean-free-path of the sputtered atoms at least as great as the envelope dimensions, i.e., the tube diameter, so the deposition of these atoms will proceed as in a high vacuum environment with no diffusion problems. The plasma density is kept as high as possible by operating the main discharge with fairly large current, e.g., 2 to 10 amps, and/or by the magnetic field of magnet 59 which causes electrons passing through the annular opening 61 thereof toward the anode 56 to spiral and thereby increase the ionization probability.

The sputtered atoms leave the material 122 in the ground state, i.e., in their non-excited state, and enter the space between the target electrode 58 and the magnet 59. To facilitate further explanation of this invention said space will hereinafter be referred to as the emission spectrum space. The magnet 59 besides concentrating the plasma density in the emission spectrum space causes electrons in the plasma to spiral. These spiraling electrons collide with the aforementioned sputtered atoms and thereby excite said atoms to higher energy levels. The raising of the energy levels of said sputtered atoms to higher energy levels by causing said sputtered atoms to collide with spiraling electrons is another step in the present invention. As these atoms which have been raised to said higher energy levels begin to return to their more stable lower energy levels, they give off energy which provides an emission spectrum of the material 122. Once the emission spectrum of the material 122 is established it can, through the observation window, either be photographed or scanned with monochromator and a photomultiplier as is well known in the art.

With a properly shaped magnetic field, one can create a plasma or ion beam which can be focused on a small spot of the target material. By making electrode 58 in the form of a rotatable turntable with means (not shown) for controlling the rotation thereof several materials can be arranged on said turntable and studied without having to open the unit, i.e., without removing the envelope 12' from the tank assembly 14.

As is well known in the art the water temperature between the walls 16 and 20 can be used to control the mercury vapor pressure in the tube (at 15 C.; 1 micron).

When it is desired to work in a gas plasma other than mercury e.g., a plasma of a noble gas like He, Ne, A, Kr and Xe, one can freeze out the mercury ions from the anode portion of the tube, i.e., the portion of the tube enclosed by the envelope 12 above the grid 50, by introducing liquid nitrogen into the re-entrant cold trap cavity 39 of the envelope 12 and then admit the gas which is desired through the gas inlet passage 35. Mercury atoms which then try to enter the anode portion of the tube from below through the hole in the magnet 59, are mostly ionized and then in the electric field are drawn back downward into the cathode section of the tube. This efiect creates a pumping action which helps to keep the anode part of the tube, when operated with a noble gas at high current, comparatively free of mercury. In this way one can operate a gas discharge other than mercury in the main anode portion of the tube, but retain the very convenient mercury pool type cathode which is an efiicient and inexhaustable source for electrons ready for operation immediately after pump down. In both mercury and noble gas discharges, the background pressure of reactive gases is down to the to 10- mm. mercury range.

The shutters like 112 and 116 protect the observation windows from sputtered deposits during the sputtering of the material which is to be spectroscopically studied. Shutters 112 and 16 make it possible to keep the observation windows comparatively free of sputtered deposits for a long time, because they are normally position between the sputtering space located between the electrode 58 and the magnet 59 and said observation Windows. In an arrangement like shown in FIGURES 1 and 2 when one desires to study the emission spectrum present in said space the shutter 112 is raised to its dotted line position. In an arrangement like FIGURES 4 and 5 said space is viewed through the mirror 118. For the purpose of emission spectrum analyses the reflective characteristic of said mirror do not appear to be adversely affected by sputtered deposits thereon and the shutter 116 keeps the observation window located behind said shutter comparatively free of sputtered deposits for a long time.

By Way of specific example of the practice of the herein described method with employment of the above described tube, a nickel chromium alloy containing aluminum and silicon was immersed in the region of high plasma density as the material 122 to be spectroscopically studied. This material was connected to electrode 58 and positioned 10 cm. above the grid 50. This material was then subjected to cleaning ionic bombardment for about 10 minutes under conditions of approximately 300 volts ion energy and 50 ma./cm. ion current density. After the ionic bombardment cleaning period and without interruption the emission spectrum then present in the space between the electrode 58 and the magnet 59 was photographed by means of a spectrograph. This photograph provided a picture of the emission spectrum of the material being analyzed. As is well known in the art said picture was then compared with standard pictures of emission spectrum of nickel, chromium, aluminum and silicon to determine how much, if any, of said elements were present in the material 122.

In view of the principles set forth herein, I have shown some of the ways of carrying out the present invention and some of the equivalents which are suggested by these disclosures.

Now, therefore, I claim:

1. A device for producing an emission spectrum of a target material, which comprises, a low pressure high density plasma discharge means including a plurality of electrodes for controlling said plasma discharge, a target material supported adjacent to at least a pair of said electrodes and adapted to 'be bombarded by said plasma discharge for sputtering off atoms of said material, and magnetic means for concentrating and accelerating electrons of said plasma which electrons excite the sputtered atoms from a stable lower energy level to an unstable higher energy level, so that when said sputtered atoms lose energy and return to said stable lower energy level said emission spectrum is produced.

2. A device for producing an emission spectrum of a target material, which comprises, a low pressure high density plasma discharge means including a plurality of electrodes for controlling said plasma discharge, said tar- 1 get material supported between a pair of said electrodes for bombardment by said plasma which causes atoms of said target material to sputter oil, and magnetic field means in the region of said material for concentrating and accelerating electrons of said plasma in a spiral path to excite the sputtered atoms to an unstable higher energy level, whereby said emission spectrum is produced when the excited atoms return to a stable lower energy level.

3. A device for producing an emission spectrum of a material, which comprises: a tank assembly, said tank assembly including an envelope, an electrode for holding said material in a given position in said envelope, means for establishing a gas discharge plasma including ions and electrons in said envelope, and means for biasing said electrode to bombard said material with said ions and to sputter atoms of said material into an emission spectrum space adjacent to said material; and a magnet positioned in said enclosure adjacent to said material held by said electrode in said given position, said magnet being effective to concentrate and accelerate said elec trons for bombardment of said sputtered atoms in said emission spectrum space so that said sputtered atoms are excited to an unstable higher energy level upon said electron bombardment and produce said emission spectrum upon loss of energy and return to a stable lower energy level.

4. A device as set forth in claim 3 which includes an observation window in said envelope through which said emission spectrum space can be viewed.

5. A device as set forth in claim 3 which includes an observation window in said envelope through which said emission spectrum space can be viewed from outside said envelope, and a shutter disposed in said envelope between said window and said space to protect said window from sputtering deposits.

6. A device as set forth in claim 5 in which said shutter can be controlledly moved from the optical path between said window and space.

7. A device as set forth in claim 5 which includes a mirror means angularly disposed in said envelope to provide an optical path between said window and said space.

8. A method for producing an emission spectrum of a material which comprises immersing said material in a gas discharge plasma of high density established in a chamber, subjecting said material to selective and accelerated ionic bombardment so that atoms of said material are sputtered away from said material, exciting said sputtered atoms to higher unstable energy levels, and obtaining an emission spectrum of said material when the said excited sputtered atoms lose energy and return to more stable lower energy levels.

9. The method for producing an emission spectrum of a given material, which comprises:

establishing a gas discharge plasma including electrons and ions in a chamber having an emission spectrum space; mounting said given material in said chamber adjacent to said emission spectrum space; bombarding said given material with said ions to sputter atoms of said given material into said emission spectrum space; and magnetically moving said electrons into collision with said sputtered atoms of said given material in said emission spectrum space to excite said sputtered atoms from a stable energy level to an unstable, higher energy level and to render said atoms effective to produce said emission spectrum upon loss of energy and return to said stable energy level. 10. A method for producing an emission spectrum of a material which comprises immersing said material as a separate electrode in a low pressure gas discharge plasma of high density established between two other electrodes, subjecting said material to ionic bombardment to elfect the removal of contaminants therefrom, subsequent subjecting said material to selective and accelerated ionic bombardment so that atoms of said material are sputtered away from said material magnetically, concentrating said plasma in the region of said sputtered atoms and causing electrons in the plasma in the region of said sputtered atoms to move in a spiral path rather than move in straight lines, eifecting through the collisions of said accelerated ion bombardment and said spiral electrons an excitation of said sputtered atoms to unstable higher energy levels, and obtaining an emission spectrum when the said excited sputtered atoms of said material return to lower energy levels.

11. A method for producing an emission spectrum of a material which comprises immersingvsaid material as a separate electrode in a low pressure gas vdischarge plasma of high density established between an anode and a cathode, making said material negative with respect to said anode and thereby subjecting said material to ionic bombardment to effect the removal of contaminants therefrom, subsequently and while still maintaining said material negative with respect to said anode subjecting said material to selective ionic bombardment so that atoms of said material are sputtered away from said material, keeping the plasma density in the region of said sputtered atoms as high as possible, causing electrons moving through said region of sputtered atoms to pass through a magnetic field to move said electrons into collision with said sputtered atoms so that said sputtered atoms are excited to a higher unstable energy level, and obtaining an emission spectrum of'said material upon lossvof energy by said excited atoms.

12. A method for producing an emission spectrum of a material which comprises immersing said material as a separate electrode in a low pressure gas discharge plasma of high density established between two other electrodes, subjecting said material to ionic bombardment to effect the removal of contaminants therefrom, subsequently subjecting said material to selective ionic bombardment so that atoms of said material are sputtered away from said material, keeping said gas pressure low enough so that the mean-free-path of said sputtered atoms isat least as great as the dimensions of the device in which said plasma is established, and accelerating electrons in said plasma and causing said electrons to collide with said sputtered atoms to excite said sputtered atoms to an unstable higher energy level so that said sputtered atoms are conditioned for emission of said emission spectrum upon loss of energy.

References Cited by the Examiner UNITED STATES PATENTS 2,258,149 10/41 Schutze 25049.5 2,271,666 2/42 Smith 3l3l61 X RALPH G. NILSON, Primary Examiner. 

1. A DEVICE FOR PRODUCING AN EMISSION SPECTRUM OF A TARGET MATERIAL, WHICH COMPRISES, A LOW PRESSURE HIGH DENSITY PLASMA DISCHARGE MEANS INCLUDING A PLURALITY OF ELECTRODES FOR CONTROLLING SAID PLASMA DISCHARGE, A TARGET MATERIAL SUPPORTED ADJACENT TO AT LEAST A PAIR OF SAID ELECTRODES AND ADAPTED TO BE BOMBARDED BY SAID PLASMA DISCHARGE FOR SPUTTERING OFF ATOMS OF SAID MATERIAL, AND MAGNETIC MEANS TO CONCENTRATING AND ACCELERATING ELECTRONS OF SAID PLASMA WHICH ELECTRONS EXCITE THE SPUTTERED ATOMS FROM A STABLE LOWER ENERGY LEVEL TO AN UNSTABLE HIGHER ENERGY LEVEL, SO THAT WHEN SAID SPUTTERED ATOMS LOSE ENERGY AND RETURN TO SAID STABLE LOWER ENERGY LEVEL SAID EMISSION SPECTRUM IS PRODUCED.
 8. A METHOD FOR PRODUCING AN EMISSION SPECTRUM OF A MATERIAL WHICH COMPRISES IMMERSING SAID MATERAIL IN A GAS DISCHARGE PLASMA OF HIGH DENSITY ESTABLISHED IN A CHAMBER, SUBJECTING SAID MATERAIL TO SELECTIVE AND ACCELERATED IONIC BOMBARDMENT SO THAT ATOMS OF SAID MATERIAL ARE SPUTTERED AWAY FROM SAID MATERIAL, EXCITING SAID SPUTTERED ATOMS TO HIGHER UNSTABLE ENERGY LEVELS, AND OBTAINING AN EMISSION SPECTRUM OF SAID MATERIAL WHEN THE SAID EXCITED SPUTTERED ATOMS LOSE ENERGY AND RETURN TO MORE STABLE LOWER ENERGY LEVELS. 